(1) Field of the Invention
This invention relates to methods for locating a magnetic implant, and more specifically, to a method for locating a magnetic object being guided, in a surgical application, by the field of a source magnet.
(2) Description of Related Art
In the field of surgery, there exists a need to control the orientation, forces, and/or motion of internally implanted devices. One method that has been used to control such implanted devices is the application of a magnetic field from an external magnet. In this method, the magnetic field acts upon the implanted device, which itself comprises magnetic material, which may be in the form of a permanent magnet. In accordance with prior art practice, a physician surgically implants the device comprising magnetic material and then guides the position of the implanted device by moving an external permanent magnet and observing the resultant movement directly with an X-ray fluoroscope. Examples of the prior art may be found in a review article by Gillies et al., xe2x80x9cMagnetic Manipulation Instrumentation for Medical Physics Research,xe2x80x9d Rev. Sci. Instrum. 65, 533 (1994). See also McNeil et al., xe2x80x9cFunctional Design Features and Initial Performance Characteristics of a Magnetic-Implant Guidance System for Stereotactic Neurosurgery,xe2x80x9d IEEE Trans. Biomed. Engrg., 42, 793 (1995); Tillander, xe2x80x9cMagnetic Guidance of a Catheter with Articulated Steel Tip,xe2x80x9d Acta Radiologa 35. 62 (1951); Frei et al, xe2x80x9cThe POD (Para-Operational Device) and its Applications,xe2x80x9d Med. Res. Eng. 5,11 (1966); U.S. Pat. No. 3,358,676 to Frei et al., issued Dec. 19, 1967, entitled xe2x80x9cMagnetic Propulsion of Diagnostic or Therapeutic Elements Through the Body Ducts of Animal or Human Patientsxe2x80x9d; Hilal et al., xe2x80x9cMagnetically Guided Devices for Vascular Exploration and Treatment,xe2x80x9d Radiology 113, 529 (1974); Yodh, et al., xe2x80x9cA New Magnet System for Intravascular Navigation,xe2x80x9d Med. and Biol. Engrg., 6, 143 (1968); Montgomery et al., xe2x80x9cSuperconducting Magnet System for Intravascular Navigation,xe2x80x9d Jour. Apppl. Phys. 40, 2129 (1969); U.S. Pat. No. 3,674,014 to Tillander, issued Jul. 4, 1972, entitled xe2x80x9cMagnetically Guidable Catheter-Tip and Methodxe2x80x9d; and U.S. Pat. No. 3,794,041 to Frei et al., issued Feb. 26, 1974, entitled xe2x80x9cGastrointestinal Catheter.xe2x80x9d The full content of each of the cited documents are herein incorporated by reference in their entirety.
Obviously, the above-described technique requires the physician to react to the movement of the implanted device. Determination of this movement can be a problem, because the implanted device can, in general, move in three-dimensional space inside the patient. With prior art hand-held magnets, the only feedback the surgeon could have was his observation of motion of a magnetic implant by x-ray or ultrasonic imaging in response to his movement of the magnet. Usually, fluoroscopic imaging is employed. However, fluoroscopic imaging can be subject to interference from the magnet, itself. In difficult interference situations, it is difficult without proper imaging guidance to provide even a reasonable guess as to a correct direction for the magnet axis to obtain field alignment with the intended path. The large electromagnet of Yodh et al. (supra) is one attempt to minimize the xe2x80x9cblindnessxe2x80x9d of the approach just described, but the Yodh et al. approach still relies on operator judgment and vision, and is subject to such error. While multiple coil arrangements such as the magnetic stereotaxis system (MSS) described in McNeil et al. (supra) can be used to provide:such guidance, it is difficult in such systems to provide a combined guiding force and force-applying field gradient in the same desired direction.
U.S. Pat. No. 5,558,091 issued Sep. 24, 1996 to Acker et al., which is hereby incorporated by reference in its entirety, discloses a magnetic position and orientation determining system using magnetic fields. By monitoring field components detected at a probe during application of the fields, the position and orientation of the probe in the field can be determined. A representation of the probe can be superimposed on a separately acquired image of the subject to show the position and orientation of the probe with respect to the subject. Although the devices and methods disclosed in this patent can determine the location of an implant, the magnetic fields used are so small as to not exert any significant, perceptible forces on magnetic materials in the sensing region. There is no disclosure or suggestion to use a magnetic field to both align and/or guide an implanted probe as well as determine its location via an externally applied magnetic field, nor is there any suggestion to place strongly magnetizable materials or permanent magnets in a seed or on a probe, adjacent to a magnetic sensor, in such a manner as to allow accurate determination of location using the magnetic sensor in the immediate vicinity of the seed.
Clearly, both operation time and risk to a patient could be reduced if an apparatus and method were available to more accurately and reliably locate, as well as guide, orient, and/or move a magnetic surgical implant. (For present purposes, when reference is made to xe2x80x9cguidingxe2x80x9d an implant, it should be assumed that this may also refer to xe2x80x9corientingxe2x80x9d an implant, as well.) Preferably, while such apparatuses and methods may allow the use of x-ray, ultrasonic, or fluoroscopic imaging devices, they should not require such imaging devices to provide the location of the implant. In addition, it would be advantageous if the location method would not require the addition of magnetic field creating devices, such as is required by the Acker et al. patent, and which might further increase the interference with the location and operation of a guiding magnet. If the location can be obtained without the use of such additional imaging and/or locating devices, the external magnet or electromagnet (or magnets or electromagnets) used for guiding the magnetic implant may be provided with a larger, unobstructed range of motion. It would also be advantageous if the location can be obtained without being subject to interference from the magnet itself, as occurs or can occur with many common fluoroscopic imaging systems.
There is thus provided, in accordance with a first aspect of the invention, a magnetic surgical implant comprising a flexible probe having a magnetic seed mounted at a distal end thereof and a magnetic field sensor mounted near the magnetic seed and in a fixed relationship thereto so that as an external magnetic field acts on the magnetic seed to guide or propel it through a patient""s body, the magnetic field sensor is also acted upon and provides an output that can be correlated with the position of the magnetic seed.
This embodiment of the invention may include a 3-axis magnetometer probe as the magnetic field sensor, which may be mounted within the flexible probe. Signals from the sensor may be conducted from the magnetic field sensor through the flexible probe by at least one sensor conductor. The probe itself may be a catheter or an endoscope.
There is also provided, in accordance with another aspect of the invention, a device for providing position data for a magnetic surgical implant having a magnetic field sensor which is guided or propelled through a patient""s body by an external magnet, said device comprising a current source for supplying a modulated electrical current to the external magnet to thereby generate an oscillating magnetic field component, a demodulator coupled to the magnetic field sensor and responsive to a magnetic field direction and magnitude of the oscillating magnetic field component to provide a signal indicative of the oscillating magnetic field component at the location of the magnetic field sensor, a memory containing a representation of a relationship of spatial locations to a magnetic field pattern produced by the external magnet, and a data processor coupled to the demodulator and the memory and configured to compute a spatial location of the magnetic field sensor as a function of the signal indicative of the oscillating magnetic field component.
In accordance with yet another aspect of the invention, a method for locating a magnetic implant is provided, comprising the steps of (a) surgically implanting a magnetic implant including an associated magnetic probe in a patient; (b) applying a modulated magnetic field from an external electromagnet; (c) detecting signals from the magnetic probe resulting from the modulated magnetic field; and (d) calculating the relative location and orientation of the magnetic probe, and therefore, the magnetic implant, with respect to the electromagnet from the detected signals from the magnetic probe.
Additional steps to the above-described method may be added, in accordance with the invention. Such steps may include locating the patient and the electromagnet so that an indication of the magnetic probe, and therefore, the magnetic implant, is displayed. An additional step of superimposing this indication over a preoperative image of the patient may also be included.
It is thus an object of this invention to provide a method and apparatus for locating a magnetic object which is being guided by the field of a source magnet, using the field of the source magnet itself to locate the object.
It is a further object of this invention to provide a method and apparatus for accurately and reliably locating a magnetic surgical implant without requiring x-ray, ultrasonic, or fluoroscopic imaging devices.
It is another object of this invention to provide a method and apparatus for accurately and reliably locating a magnetic surgical implant by using a magnet that simultaneously serves the functions of guiding a magnetic implant and locating it.
It is yet a further object of the invention to provide a method and apparatus for locating a magnetic surgical implant that permits a wide range of unobstructed motion to be provided to an external magnet or electromagnet for guiding the magnetic surgical implant.
It is a still further object of the invention to provide a method and apparatus for locating a magnetic surgical implant that is not subject to interference caused by the presence of the external magnet.